1. Field of the Invention
The present invention relates to an impact strength modifier for polyvinyl chloride (PVC) and a method of preparing the impact strength modifier, and more particularly, to a graft copolymer of alkyl (meth)acrylate-a conjugated diene monomer-an ethylenically unsaturated aromatic compound (hereinafter, referred to as “MBS based graft copolymer”), useful as an impact strength modifier for PVC resin, and a method of preparing the graft copolymer.
2. Description of the Related Art
PVC resin comprises at least 50% of vinyl chloride. Since PVC resin is very weak to impact, research has been conducted on methods to overcome this problem. Methods of increasing impact resistance of PVC resin include using a graft polymer as impact strength modifier for PVC resin, which is prepared by grafting monomers, such as styrene, methyl methacrylate, or acrylonitrile on rubber latex based on butadiene. However, although this method provides an effect of modifying impact strength of the PVC resin, the resulting products have low transparency and whitening occurs during formation of sheets of the PVC resins.
It is known that physical properties of MBS based graft copolymers depend on amounts of the respective monomers to be grafted, polymerization method, and an amount and particle diameters of rubber latex used as a substrate, etc. In order to increase impact strength, the amount and the particle diameters of the rubber latex used as a substrate are generally increased. However, when such a rubber latex is included in PVC resin, since more light scatters due to the increase of the particle diameters of the graft copolymer used as an impact strength modifier, the transparency of the PVC resin decreases. In addition, when a difference between a refractive index of the PVC resin particles and a refractive index of the graft copolymer particles is large or when a binding force between the MBS based graft copolymer and the PVC resin is weak during deformation and thus, micro-voids are easily formed, rather a degree of whitening may increase and the impact strength may decrease.
Much research has been conducted on an amount and a size of rubber particles, a graft polymerization method, and a graft composition, etc. to prepare graft copolymer particles having excellent impact resistance, transparency, and resistance to whitening. In particular, it is well known that when the use of rubber latex is limited according to its application, the amount of the monomers to be grafted and the graft polymerization method have a significant effect on the transparency and impact strength of a product.
In general, MBS based graft copolymers are typically used as additives to increase impact strength, resistance to whitening, processability, and optical properties of PVC resin. MBS based graft copolymers are generally prepared by grafting alkyl (meth)acrylate and an ethylenically unsaturated aromatic compound onto a rubber latex of a conjugated diene monomer-an ethylenically unsaturated aromatic compound such as styrene-butadiene rubber latex, using emulsion polymerization.
Factors having effects on the physical properties of MBS based graft copolymer include the amounts of the respective monomers to be grafted and the polymerization methods, and the most important factor is the physical property of a rubber latex used as a substrate of the MBS based graft copolymer. In particular, it is known that the size and amount of the rubber latex and a compositional ratio of monomers have direct effects on the optical and physical properties of the PVC resin.
U.S. Pat. No 4,352,910 by Katto et al. describes a method of preparing an MBS based graft copolymer well balanced between impact strength and resistance to whitening by controlling a glass transition temperature of a rubber latex using a multi-step polymerization.
However, improvement of color properties and low-temperature impact strength of PVC resins using MBS based graft copolymers is not described in conventional methods.
Thus, there is a need for a graft copolymer capable of improving color property and low-temperature impact strength of PVC resin and a method of preparing the graft copolymer.